Bookbinder's buy (4)
I believe the answer is:
glue
'bookbinder's buy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for glue that I've seen before include "Paste" , "Adhesive substance" , "agent with sticking power" , "Adhesive stuff" , "Stuff that sticks firmly" .)